Sound absorption



Jan; 10, 1 950 c, MANNAL 2,494,343

SOUND ABSORPTION Filed April 18, 1945 'liilllil'fi VIIIIIII VIII! 'IIIII/ 'IIII/ 'III/ 5 Inventor: Clifford Manna],

Hi Attorney mama Jam10,1950

SOUND ABSORPTION Clifford Manual, Lee, Mam, assignor-to General Electric Company, a corporation of New York Application April 18, 1945, Serial No. 589,015

1 Claim. 1

This invention relates to sound absorption and more particularly to improvements in sound absorbents suitable for use in liquids.

It has been known for some time that one method of sound reduction in air is to present a cellular structure or capillary bed to the oncoming sound wave. The walls of the cells or capillaries provide a large surface area generally parallel to the air vibrations and the friction between the vibrating air and this large stationary surface damps the vibration and converts the sound energy into heat. All known sound absorbents for air work on this principle. Such cellular structures or capillary beds are made of organic material, such as cork, felt or cellulosic material, and also of inorganic material, such as mineral wool. However, such materials which effectively absorb sound in air are ineffective to absorb sound in liquids.

There are many situations where an objectionable amount of sound is transmitted through a liquid. For example, most of the transformers used in electric power systems are enclosed in metal tanks which are substantially filled with an insulating and coolin liquid. The alternating magnetization of the core of these transformers causes the surface of the core to vibrate in and out. These vibrations are communicated to the surrounding liquid and travel as sound waves through it to the walls of the enclosing tank which in turn is caused to vibrate and send out sound in the surrounding air. The result is an unpleasant hum which is quite objectionable in certain locations, such as residential communities. The problem of finding an effective sound absorbent suitable for use in liquids has therefore been an important one.

I have discovered that the reason ordinary sound absorbents, which are effective in air, are not efiective in liquids is that they saturate with the liquid. Since the walls of the cells or the capillary meshes become filled with the liquid, they effectively lose their identity or difference in density so far as the liquid filling the cells or capillaries is concerned and consequently sound energy is no longer dissipated by being passed through this structure. Theoretically, therefore, the solution of the problem is to make the cellular structure of material which will not become saturated with the liquid. Practically. however, the matter is not that simple, particularly in the case of stationary induction apparatus, such as transformers and reactors. In the first place. the nonsaturated cellular or mesh-like structure must be relatively inexpensive so that the idea of honeycombing a thick metal plate with myriads of tiny thin walled holes by any known process would be prohibitively expensive. In addition, the structure must be permanent and easily applied. Furthermore, in the case of stationary induction apparatus, it must not contaminate the liquid or upset the cooling system.

In accordance with a preferred embodiment of my invention I use multiple layers of ordinary wire mesh screening without in any way aligning the openings in adjacent layers. In this way the effective diameter of the passageways through the structure is decreased and the effective length of the passageways, being zig-zagged, is greater than the straight line thickness through the layers. Both of these factors tend to increase the sound absorbing effectiveness of the structure.

An object of the invention is to provide improvements in sound absorption.

Another object of the invention is to provide a novel sound absorbent structure which is suitable for use in liquids.

A further object of the invention is to provide an improved arrangement for quieting stationary induction apparatus.

The invention will be better understood from the followin description taken in connection with the accompany drawing and its scope will be pointed out in the appended claim.

In the drawing Fig. 1 is a sectional view of a transformer provided with an embodiment of my invention, Fig. 2 is a plan view partly broken away of Fig. 1, Fig. 3 is an enlarged detail view of a modified form of sound absorbing mesh structure, Fig. 4 is a view similar to Fig. 2 of another modification of the invention, and Fig. 5 is a detailed view, generally similar to Fig. 1, of a further modification of the invention.

Referring now to the drawing and more particularly to Fig. 1, there is shown therein a transformer comprisin a core I and coils 2. This transformer is surrounded by a conventional metal tank 3 which is substantially filled up to the level 4 with an insulating and cooling liquid 5, such as oil or askarel. The core i is separated from the bottom of the tank 3 in any suitable manner, such as by resting it on blocks of wood 8.

For absorbing sound emanating from the transformer and moving outwardly toward the walls of the tank 3 I surround the transformer with a multiple layer cylinder of wire mesh screening 1 which may be made of any suitable material, such as copper. be wound spirally around the transformer as This screening may 3 many times as desired so as to form a cylindrical structure extending from the bottom of the tank at least up to the surface of the liquid.

In tests which I have conducted on a 60-cycle per-second transformer in which copper wire screening 1 was wrapped around the transformer so as to provide thirteen layers and in which the size of the screening was 16 mesh (corresponding to ordinary fly screen) a. noise reduction of six decibel or about forty per cent wasobtained.

The amount of sound absorbed. is generally proportional to the number of layers of screening and it is also a function of the size or mesh of the screening. However, a theoretical determination of the optimum size or mesh of the screening is exceedingly complex because the problem involves so very many parameters, such as shape of the sound wave front, its angle of incidence, density and viscosity of the liquid, temperature of the liquid and frequency of the sound, that no conclusive results have been obtained. However, if the problem is simplified by considering the structure to have a certain definite thickness and to be perforated by parallel holes the sum total of whose area is half the total wall area it can be shown that the optimum diameter of the cells or capillaries is somewhere between .01 and .05 centimeter for a thickness ranging between ten and twenty centimeters for liquids of the density of conventional transformer cooling liquidsf Once the sound vibrations get into the cellular structure, the smaller the cell or duct diameters the greater the absorption or attenuation of the sound, but when the diameter of the ductsis too small, the structure reflects an excessive amount of the sound and too small a proportion enters the structure. However, all things considered, it is better to have the diameter too small than too large because sound which is reflected by the structure must again return to it so that eventually it will be absorbed by the structure.

It will of course be obvious that the structure 1 shown in Figs. 1 and 2 only absorbs sound which otherwise would reach the side walls of the tank and does not absorb the sound which goes directly from the transformer to the top and bottom of the tank. Of course aflat multilayer screen could be mounted transversely so as to absorb the sound rising upwardly from the transformer toward the top of the tank but this is relatively unimportant because of the air cushion between the top of the tank and the top level 4 of the liquid which in itself is a relatively poor sound conductor.

As the transformer is connected mechanically to the bottom of the tank through the blocks 5, the use of screening between the bottom of the tank and the bottom of the transformer would not be especially effective.

In the modification shown in Fig. 3 the layers of screening starting with the innermost layer, that is to say, the layer closest to the transformer, and progressing outwardly have successively finer mesh. In this manner there is minimum reflection by the structure I of the onooming sound wave from the transformer so that the sound gets well into the structure before appreciable absorption begins to take place,

but as the size of the holes in each successive layer gets smaller and smaller, the absorption increases progressively.

It is not essential that the surface of the screening forming the sound absorbing structure have its surface substantially perpendicular to The edgewise or radially disposed groups 1 are preferably backed up by. a sound reflector member 9. This is preferably made of imperforate insulating material, and maybe in the form of a cylinder, as indicated.

In the modification shown in Fig. 5 layers of organic material, such as cotton cheesecloth, gauze or other suitable material are placed between alternate layers of the wire screen. Al-

' though these cloth layers become saturated with the dielectric and cooling liquid which substantially fills the tank 3, they are nevertheless held firmly in place by being backed up on both sides by the rigid non-saturated layers of metal screen. In this manner the sound absorbing properties of the structure may be increased as the alternate layers of metal screen and cloth offer a higher resistance to sonic vibrations of the liquid.

While there have been shown and described particular embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention, and, therefore, it is aimed in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure b Letters Patent of the United States, is:

In an electrical induction apparatus, a core and coil structure which produces sound waves during operation, a casing surrounding said core and coil structure, a dielectric liquid within said casing and surrounding said core and coil structure, and means for minimizing the transfer of sound waves from the core and coil structure through the liquid including a plurality of layers of a wire mesh between said core and coil structure and said casing, said plurality of layers of mesh having progressively changing size openings with the greatest size of openings on the inside of the structure so that the energy of the sound waves in passing through the openings in the mesh will be dissipated.

CLIFFORD MANNAL.

REFERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,127,250 Humm Feb. 2, 1915 1,580,811 Burnham Apr. 13, 1926 1,589,408 Maxfield June 22, 1926 1,641,374 Chryst Sept. 6, 1927 1,838,645 Voigt Dec. 29, 1931 1,846,887 Matthews Feb. 23, 1932 1,912,544 Slidell June 6, 1933 1,957,245 Burnham May 1, 1934 1,968,456 Kliefoth July 31, 1934 2,394,461 Mason Feb. 5, 1946 FOREIGN PATENTS Number Country Date 404,148 France of 1909 

